The collision between Andromeda and our galaxy has started
Let's go back for a while to 1929. That was pretty the year that changed our world.
One of the greatest astronomers of all time discovered one of
the most mind-blowing laws of the universe.
What he found was that the entire universe was expanding, and it was doing so according to the
following empirical formula:
This almost magic formula was saying that the more an object was distant from us,
the faster it was moving away from us. Since then, astronomers from all over
the world made a lot of effort in order to insert this new amazing law
in a good physical framework, and today such an important law is known as the Hubble's Law.
The H zero constant in the formula is the famous Hubble constant, and yes...it changed
our understanding of the universe...forever. Edwin Hubble's contribution was so important
that today, almost everyone knows the universe is expanding.
Later observations in the years continued to confirm this empirical law, and today one of the
most challenging problems to solve is to find a more precise value of the Hubble constant.
Its value, as of today, is set at about 71 km per second per megaparsec.
I know what you're thinking: why is he talking about Hubble's law? This video is supposed to
be about the Milky Way and Andromeda collision! Don't worry, I didn't forget. The Hubble's law
will be important for us to understand why we can safely say that Andromeda is coming...toward us.
Follow me in this video to get to know why about galaxies collisions, and to know what
will happen to our Galaxy, the Milky Way, in the next billions of years according to models.
So what Hubble actually discovered was a tight
relation between the redshift of other galaxies and their distance from us.
This of course translated into the well-known Hubble's law we've already mentioned.
But what does it mean? What was this “tight relation”?
He found that the more distant an object was, the more shifted its spectral lines were.
And they were shifted to the red part of the electromagnetic spectrum.
Usually, astronomers don't take photos. They analyze the stellar spectrum,
that is, they study the division of light coming from a stellar source. This light
is characterized by lines that cross the whole spectrum, and these lines contain
crucial pieces of information about stellar composition, density, gas kinematic and so on.
That was an empirical law that turned out to be very useful, because one just needed to know the
correct value of the Hubble constant, in order to find the distance of any object in the sky.
You had a telescope? So you had the redshift, that is the displacement of lines with respect to the
laboratory obtained lines. You had the redshift? Hubble's relation was telling you the distance of
your source. It seemed an easy trick, too good to be true. So a doppler shift towards the red
part of the spectrum means the object is getting away from us. On the opposite side, a blue-shift
in the lines of stellar objects means...it is approaching us towards the line of sight.
It is now that we are finally ready to talk about galaxies collisions.
Soon after Hubble, astronomers began to study the objects in the sky and take
as much redshift information they could. Because of the expansion of the universe,
the vast majority of objects in the deep universe were found
indeed to go away from us, at some speed determined by Hubble's law.
But...something strange was happening: some objects didn't seem to care about Hubble's law:
some objects were simply...going towards us. The question was: why is that?
Apparently, these objects were violating Hubble's law...did they?
Well, the answer here is that nature is not a joker. Everything has an explanation,
even the weirdest behaviour can be explained if you study and think more about it.
This strange behaviour that characterized some stellar objects was due to the gravitational
attraction of the objects. I mean, once we know Hubble's law, we can't suddenly forget about
Newton's gravitational law. What I am trying to say is that, yes, Hubble's law isn't fake,
but it just “works better” with objects that lie at a very long distance from us.
For nearer objects, we found that the gravitational pull is stronger
than the acceleration rate of the expanding universe. This means that, from time to time,
in a world of redshifted objects, we can happen to see some blueshifted ones.
These, as we've already said, are objects that are coming toward us.
It is no magic! It's just gravitational attraction that overcomes the expansion of the universe.
Well, when astronomers pointed their telescopes to Andromeda Galaxy,
they clearly saw its spectral lines were blue shifted.
This could only mean one thing: our galaxy was on a collision course.
In roughly 4.5 billion years’ time, the Milky Way will smash into the rapidly
approaching Andromeda Galaxy, and astronomers are still attempting to predict what it will
be like when the two galaxies collide. Anyway, such doppler shift measurements
can only tell you about the motion directly towards or away from the observer. Working out
if we were in for a direct hit or a near miss, therefore, required careful study of the motion
of Andromeda’s satellite galaxies, and they reveal Andromeda’s trajectory makes collision inevitable.
Don't worry, there's no need to be scared, and I'll explain why to you later in this video.
For now, let's talk about how this collision will look like.
Plugging this information on the relative velocities, estimates of the masses of the
galaxies and details of their structure into their model, the authors of an interesting paper on this
galactic collision have watched the event unfold. Though the details depend to some extent on
the galactic structure and in particular on assumptions we make about the haloes of dark
matter that contain the visible discs of the Milky Way and Andromeda, the general picture is clear.
First. There will be a series of close passes lasting something like billions of years.
At each passage, the structure of the two galaxies will be distorted by tidal forces
of gravitational potential. All of these passes will end up in a
beautiful merger of the andromeda galaxy and the Milky Way galaxy.
As of today, astronomers observed a bunch of ongoing collisions between galaxies.
For example, take a look at this image. It shows the object known
as an NGC 34. It definitely seems this galaxy looks a bit weird in shape.
The main cause for this galaxy’s odd appearance lies in its past. If we were able to reverse time
by a few million years, we would see two beautiful spiral galaxies on a direct collision course.
When these galaxies collided with one another, their intricate patterns and spiral arms were
permanently disturbed. This image shows the galaxy's bright centre,
a result of this merging event that has created a burst of new star formation
and lit up the surrounding gas. As the galaxies continue to intertwine and become one,
NGC 34’s shape will become more like that of a peculiar galaxy, devoid of any distinct shape.
In the vastness of space, collisions between galaxies are quite rare events, but they can
be numerous in mega-clusters containing hundreds or even thousands of galaxies.
As in the case of NGC 34, the result of the collision between Andromeda
and the Milky Way will be a new, larger galaxy, but rather than being spiral-like its forebears,
this new system ends up as a giant elliptical. An elliptical galaxy is a type of galaxy with
an approximately ellipsoidal shape and a smooth, nearly featureless image.
The study authors have named this new galaxy ‘Milkdromeda’.
What would you call it? Let us know in the comments below!
However, we already said that you don't need to be scared.
First of all, you will die long before that happens.
But even if you happen to find the long life elixir and be here in 10 billion years,
you don't need to be scared. Here's why. While the Andromeda Galaxy contains about
1 trillion (1012) stars and the Milky Way contains about 300 billion (3×1011), the
chance of even two stars colliding is negligible because of the huge distances between the stars.
For example, the nearest star to the Sun is Proxima Centauri, about
4.2 light-years (4.0×1013 km; 2.5×1013 mi) or 30 million (3×107) solar diameters away.
To visualize that scale, if the Sun were a ping-pong ball, Proxima Centauri would be a pea
about 1,100 km (680 mi) away, and the Milky Way would be about 30 million km (19 million mi) wide.
Although stars are more common near the centres of each galaxy, the average distance between stars is
still 160 billion (1.6×1011) km (100 billion mi). That is analogous to one ping-pong ball every 3.2
km (2 mi). Thus, it is extremely unlikely that any two stars from the merging galaxies would collide.
But what will be the solar system fate, according to this scenario?
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Well, predictions say that the Solar System will be swept out three times farther from
the galactic core than its current distance. They also predict a 12% chance that the Solar
System will be ejected from the new galaxy sometime during the collision.
Such an event would have no adverse effect on the system and the chances of any sort of disturbance
to the Sun or planets themselves may be remote. Also, by the time the two galaxies collide,
the surface of the Earth will have already become far too hot for liquid water to exist,
ending all terrestrial life; that is currently estimated to occur in about 3.75 billion years due
to the gradually increasing luminosity of the Sun. The paper ends on a melancholic note: the authors’
estimate of 10 billion years for the merger to reach its end is longer than in previous studies
but means that the Sun will not live long enough to find a home in the new galaxy.
Isn't it beautiful? We can predict what will happen in the future, even if we won't be able
to actually see it happen. What a time to be alive!
"This video ends here! Thanks for watching everyone! The Andromeda-Milky
Way collision is something we will miss. Do you feel sad about that?
Let us know in the comment below! See you next time on the channel!”